Airborne Transmission of COVID-19: Epidemiologic Evidence from Two Outbreak Investigations

Background: Much remains unknown about COVID-19 transmission. We evaluated potential transmission routes from two community COVID-19 outbreaks.Methods: In the first outbreak, 126 passengers took two buses (59 from Bus #1 and 67 from #2) on a 100-minute round trip to attend a 150-minute worship event. The source patient was a passenger on Bus #2. We compared risks of COVID-19 among individuals taking Bus #1 (n=60) and Bus #2 (n=67), and among all other individuals (n=172) attending the worship event. We also divided seats on the exposed bus into high- and low-risk zones according to distance to the source patient and compared COVID-19 risks in each zone. The second outbreak occurred among 30 trainees attending a 3-day workshop in several conference rooms. In both buses and conference rooms, central air-conditioners were in indoor re-circulation mode. Results: In the first COVID-19 outbreak, passengers in Bus #2 had a 41.5 (95% confidence interval [CI]: 2.6–669.5) times higher risk of getting COVID-19 compared to those in Bus #1, and 11.4 (95% CI: 5.1–25.4) times higher risk compared to all other individuals attending the worship event. Within Bus #2, passengers in high-risk zones had moderately, but non-significantly, higher risk for COVID-19 compared to those in the low-risk zones. In the second outbreak, the overall attack rate was 48.3%.Conclusion: Airborne spread of COVID-19 appears to at least partially explain the high attack rates in the exposed bus and conference rooms. Future efforts at prevention and control must consider the potential for airborne spread of the virus.